Problem: Which of these numbers is prime? ${5,\ 39,\ 78,\ 87,\ 91}$
A prime number has exactly two factors: 1 and itself. The factors of 5 are 1 and 5. The factors of 39 are 1, 3, 13, and 39. The factors of 78 are 1, 2, 3, 6, 13, 26, 39, and 78. The factors of 87 are 1, 3, 29, and 87. The factors of 91 are 1, 7, 13, and 91. Thus, 5 is a prime number.